In web development, you sometimes come across a segment of a URL called a "slug", which is human readable string of text appended to a URL to make easier to determine the content the URL points to. The slug itself if usually not actually necessary for web server to serve the correct page. For example, these two links point to the same page:
https://stackoverflow.com/questions/4230846/what-is-the-etymology-of-slug-in-a-url
https://stackoverflow.com/questions/4230846
This use of the word's use in web media is following a convention from print media, where a "slug" is an informal name given to a story before publication. Print media takes the term from printing presses, where a slug is a line of lead cast characters.
The term slug derives from the days of hot-metal printing, when printers set type by hand in a small form called a stick. Later huge Linotype machines turned molten lead into casts of letters, lines, sentences and paragraphs. A line of lead in both eras was known as a slug.
Screenplays have a similar concept, where a "slug line" is a heading at the start of each scene containing important general information about the following scene.
A slug line, also called a master scene heading, occurs at the start of each scene and typically contains 3 pieces of information: whether the scene is set inside or outside (INT. or EXT.; interior or exterior), the specific location, and the time of day. Each slug line begins a new scene. In a "shooting script" the slug lines are numbered consecutively for ease of reference.